This invention relates to security documents, that is to say, documents such as cheques, bank notes, bankers drafts, etc. which have anti-counterfeiting features provided by security printing.
For security documents it is well known to use moire patterns which are interference fringes arising from two sets-of generally parallel lines, the line sets being superimposed upon one another with their lines intersecting and mutually inclined at a small angle.
Such moire patterns have hitherto provided a satisfactory degree of security against counterfeiting, but developments in the technical apparatus available to the counterfeiter (for example, color photocopiers) have rendered them increasingly less secure. The present invention therefore seeks to improve the security provided by moire patterns, and accordingly provides, according to the first aspect, a security document, which is printed with a moire pattern formed of first and second sets of intersecting lines arranged to form one or more moire effect interference fringes, wherein the lines of at least one of the line sets vary progressively in thickness along their length.
For additional security one or both of the sets of lines may be printed in inks having anti-fraud characteristics. The anti-fraud characteristics (where provided) of the two inks may differ, or they may be the same; for example, one set of lines may be printed in a solvent-sensitive ink and the other in a water-fugitive ink. As another possibility, one or both sets of lines may be printed in an ink so as to be visually distinguishable only under special lighting for example, under ultra-violet (UV) light.